1. Field of the Invention
The present invention relates to a camera having a holographic indicator and more particularly to a camera having a holographic indicator of the type which forms a reconstructed image of a hologram on the same plane as that of an object's image formed by an image forming optical system.
The term "hologram" is used herein to mean interference fringes which are formed by an object bearing light and a reference light and recorded on a recording medium, a copy of the recorded interference fringes, a computer calculated hologram or the like.
2. Description of the Prior Art
A holographic indicator relevant to that of the present invention has been already disclosed in the specification of U.S. Pat. No. 3,633,988. In the known apparatus, a hologram is disposed at a sight window provided in the helmet for a pilot to enable him observe a far distant object simultaneously with an image reconstructed from the hologram at the far distant position.
Also, in U.S. Pat. No. 3,560,921 there is disclosed such a device which enables one to observe a gage of a vehicle and an indicating mark reconstructed from a hologram at the same time.
The subject of the present invention is entirely different from those of the above mentioned U.S. Patents. The present invention relates to the use of a holographic indicator in camera. The present invention is directed to means by which an object's image and a reconstructed image of a hologram can be formed on one and entirely the same plane at the view finder of camera and by which both the images can be observed at the same time through one and the same diopter.
When a camera is provided with a holographic indicator, it is desirable to dispose the hologram in the path of the view finder to permit a compact design of the camera. However, the arrangement of a hologram in the view finder path has a problem that the hologram has an adverse effect on the beam of light directed to the view finder. Such problem never arises in the apparatus disclosed in the above mentioned U.S. Patents specifications. In a camera it is essential that an image of the object be clearly observable through the view finder. By disposing a hologram in the light path of the view finder, the object's image observable in the view finder becomes darkened or clouded which is generally referred to as a phenomenon of darkening. The camera proposed by the present invention is featured in that the hologram disposed in the light path of the view finder never affects the image of the object when no image of the indicating mark is reconstructed and that the object's image can be observed clearly without the phenomenon of darkening.
Our prior application, now U.S. Pat. No. 4,165,930 has proposed a camera having the same feature as described above. But, the camera of the prior application uses a volume type hologram to attain the objects. in contrast, the present invention uses a relief type blazed hologram which is lower in cost and higher in mass producibility than the volume type hologram. To attain the same objects using a relief type blazed hologram, a certain particular condition is given to the diffraction grating of the hologram used according to the present invention.